1. Field of the Invention
The invention relates generally to microwave ovens and specifically to combination microwave and thermal ovens.
2. Prior Art
In microwave oven design and construction the ability to prevent microwave energy leakage is a major concern. Leakage must be prevented in microwave ovens in order to protect users from exposure to that energy. Federal standards require that the radiation leakage be less than 1 milliwatt per square centimeter at 5 centimeters distance from the microwave oven door, for the fundamental ISM band and less than 25 microvolts per meter at a at a distance of 1,000 feet for all out of band radiation.
Microwave energy will not transmit through solid metal. Therefore, the oven cavity and door in microwave ovens are formed from metal. Since the door and oven are not formed as a unified member, leakage may occur through the joint between the door and the wall of the oven cavity. In ovens where the only cooking energy is microwave, a seal can be effectively formed which will conductively seal the door and wall of the oven cavity and thereby prevent leakage of the microwave energy. In some prior art microwave ovens a flexible gasket is used to prevent that leakage. In other prior art microwave ovens a choke which causes an electrical short is employed. However, when an oven is constructed which employs both microwave energy and thermal energy for cooking, no sealing device has been designed or constructed which will effectively prevent microwave leakage. The problem results because high temperatures cause stress and expansion of the cavity walls and gasket which reduces the contact area between the gasket and cavity wall and thereby permits leakage of microwave energy. This thermal expansion problem is significantly increased when the self-cleaning feature is added to ovens employing both thermal and microwave energy. In these ovens, the temperature during the self-cleaning cycle will vary between 865.degree. and 1,000.degree. Fahrenheit. At these high temperatures the metal walls and faces of the oven are subjected to expansion and stresses not normally found in ovens only utilizing microwave energy. These stresses and expansions cause the alignment of the sealing mechanisms to change. There is no known oven which employs both thermal and and microwave energy in a self-cleaning oven which operates in the 2.450 GHz ISM band, and accordingly, there is no known device for sealing the door of such an oven.
It is acknowledged that in the prior art some microwave ovens employ chokes as electrical shorts; some employ flexible conductive gaskets, and others employ a microwave absorbing cartridge. However, none of those prior art devices employ any combination of the various techniques for sealing the door of the oven. Moreover, the present invention provides a one-half wave-length choke. This specific wave length choke is constructed such that minimum energy levels exist near the door gasket. This design prevents build-up of energy near the door gasket which could cause destruction of the gasket and thereby reduce the effectiveness of the combined choke and gasket which eliminates most of the total ISM band leakage.
The present invention overcomes the problems that exist in the prior art and provides an efficient sealing device for a combination thermal-microwave oven.